Maize (or corn; Zea mays L.) plant breeding is a process to develop improved maize germplasm in an inbred or hybrid plant. Maize plants can be self-pollinating or cross pollinating. Self-pollination for several generations produces homozygosity at almost all gene loci, forming a uniform population of true breeding progeny, known as inbreds. Hybrids are developed by crossing two homozygous inbreds to produce heterozygous gene loci in hybrid plants and seeds. In this process, the inbred is emasculated and the pollen from the other inbred pollinates the emasculated inbred. Emasculation of the inbred can be done by chemical treatment of the plant, detasseling the seed parent, or the parent inbred can comprise a male sterility trait or transgene imparting sterility, eliminating the need for detasseling. This emasculated inbred, often referred to as the female, produces the hybrid seed, F1. The hybrid seed that is produced is heterozygous. However, the grain produced by a plant grown from F1 hybrid seed is referred to as F2 grain. F2 grain which is a plant part produced on the F1 plant will comprise segregating maize germplasm, even though the hybrid plant is heterozygous.
Such heterozygosity in hybrids results in robust and vigorous plants. Inbred plants on the other hand are mostly homozygous, rendering them less vigorous. Inbred seed can be difficult to produce due to such decreased vigor. However, when two inbred lines are crossed, the resulting hybrid plant shows greatly increased vigor and seed yield compared to open pollinated, segregating maize plants. An important consequence of the homozygosity and homogeneity of inbred maize lines is that all hybrid seed and plants produced from any cross of two such lines will be the same. Thus the use of inbreds allows for the production of hybrid seed that can be readily reproduced.
There are numerous stages in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The aim is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include, for example, higher yield, resistance to diseases, fungus, bacteria and insects, better stems and roots, tolerance to drought and heat, improved nutritional quality, and better agronomic characteristics.
Choice of breeding methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pure line cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location may be effective, whereas for traits with low heritability, selection can be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences the choice of breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars and introducing transgenic events into maize germplasm. Thus, backcross breeding is useful for transferring genes for a simply inherited, highly heritable trait into a desirable homozygous cultivar or inbred line which is the recurrent parent. The source of the trait to be transferred is called the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
Each breeding program generally includes a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goals and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
The ultimate objective of commercial corn breeding programs is to produce high yield, agronomically sound plants that perform well in particular regions of the U.S. Corn Belt, such as a plant of this invention.